Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 96132363 ~ ~1 ~~ ~~ 6 ~ PCT/US96/04877
NONAZIDE GAS GENERATING COMPOSITIONS
WITH A BOILT-IN CATALYST
BACKGROLT~1D OF THE INVENTION
The present invention relates generally to gas
generating compositions used for inflating occupant safety
restraints in motor vehicles, and more particularly to nonazide
gas generants that produce combustion products having
acceptable toxicity levels in the event of exposure to vehicle
occupants.
Inflatable occupant restraint devices for motor
vehicles have been under development worldwide for many years,
including the development of gas generating compositions for
inflating such occupant restraints. Because the inflating
gases produced by the gas generants must meet strict toxicity
requirements, most, if not all, gas generants now in use are
based on alkali or alkaline earth metal azides, particularly
sodium azide. When reacted with an oxidizing agent, sodium
azide forms a relatively nontoxic gas consisting primarily of
nitrogen. Moreover, combustion of azide-based gas generants
occurs at relatively low temperatures, which allows for the
production of nontoxic inflating gases without a need for
additives to reduce the combustion temperature.
However, azide-based gas generants are inherently
difficult to handle and entail relatively high risk in
manufacture and disposal. Whereas the inflating gases produced
by azide-based gas generants are relatively nontoxic, the metal
azides themselves are conversely highly toxic, thereby
resulting in extra expense and risk in gas generant
manufacture, storage, and disposal. In addition to direct
contamination of the environment, metal azides also readily
react with acids and heavy metals to form extremely sensitive
compounds that may spontaneously ignite or detonate.
In contradistinction, nonazide gas generants provide
significant advantages over azide-based gas generants with
respect to toxicity related hazards during manufacture and
disposal. Moreover, most nonazide gas generant compositions
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typically supply a higher yield of gas (moles of gas per gram
of gas generant) than conventional azide-based occupant
restraint gas generants.
However, nonazide gas generants heretofore known and
used produce unacceptably high levels of toxic substances upon
combustion. The most difficult toxi0.~.~ases to control are the ,
various oxides of nitrogen (NOX),~and carbon monoxide (Cn).
Reduction of the levels of toxic NOx and CO upon
combustion of nonazide gas generants has proven to be a
difficult problem. For instance, manipulation of the
oxidizer/fuel ratio only reduces either the NOX or CO. More
specifically, increasing the ratio of oxidizer to fuel
minimizes the CO content upon combustion because the extra
oxygen oxidizes the CO to carbon dioxide. Unfortunately,
however, this approach results in increased amounts of NOX.
Alternatively, if the oxidizer/fuel ratio is lowered to
eliminate excess oxygen and reduce the amount of NOx produced,
increased amounts of CO are produced.
The relatively high levels of NOX and CO produced
upon combustion of nonazide gas generants, as opposed to
azide-based gas generants, are due primarily to the relatively
high combustion temperatures exhibited by nonazide gas
generants. For example, the combustion temperature of a sodium
azide/iron oxide gas generant is 969°C (1776°F), while the
nonazide gas generants exhibit considerably higher combustion
temperatures, such as 1818°C (3304°F). Utilizing lower energy
nonazide fuels to reduce the combustion temperature is
ineffective because the lower energy nonazide fuels do not
provide a sufficiently high gas generant burn rate for use in
vehicle occupant restraint systems. The burn rate of the gas
generant is important to ensure that the inflator will operate
readily and properly.
Another disadvantage created by the high combustion ,'
temperatures exhibited by nonazide gas generants is the
difficulty presented in forming solid combustion particles that '
readily coalesce into a slag. Slag formation is desirable
because the slag is easily filtered, resulting in relatively
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WO96132363 ..., t t ~ ~' t' PCllOS96I04877
clean inflating gases., In azide-based gas generants, the lower
combustion temperatures are conducive to solid formation.
However, many common solid combustion products which might be
expected from nonazide gas generants are liquids at the higher
combustion temperatures displayed by nonazide gas generants,
and are therefore difficult to filter out of the gas stream.
Therefore, a need exists for a nonazide gas generant
that can produce inflating gases in which toxic gases, such as
NOX and CO, are minimized without compromising the desired burn
rate of the gas generant.
SU~ARV Og THE TNVENTTON
The aforesaid problems are solved, in accordance with
the present invention, by a nonazide gas generating composition
which is nontoxic itself, and also produces inflating gases
upon combustion which have reduced levels of NOX and CO. The
manufacturing, storage, and disposal hazards associated with
unfired azide inflators are eliminated by the gas generants of
the invention. The reduced content of toxic gases produced
upon combustion allow the gas generants of the present
invention to be utilized in vehicle occupant restraint systems
while protecting the occupants of the vehicle from exposure to
toxic inflating gases, such as NOX and CO, which heretofore
have been produced by nonazide gas generants.
Specifically, the present invention comprises a four
component gas generant comprising a nonazide fuel, an oxidizer,
a slag former and a built-in catalyst. The nonazide fuel is
selected from the group consisting of tetrazoles, bitetrazoles
and triazoles. The oxidizer is preferably selected from the
group consisting of inorganic nitrates, chlorates, or
perchlorates of- alkali or alkaline earth metals. The slag
forming compound is selected from alkali metal oxides,
' hydroxides, perchlorates, nitrates, chlorates, silicates,
borates or carbonates, or from alkaline earth and transition
' metal hydroxides, perchlorates, nitrates, or chlorates, or from
silicon dioxide, alkaline earth metal oxides, and naturally and
synthetically manufactured magnesium and aluminum silicate
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compounds, such as naturally occurring or synthetically
formulated clay and talc.
In accordance with the present invention, the
built-in catalyst actively promotes the conversion of NOX and
CO to nitrogen gas (N2) and C02, respectively, so as to reduce
the toxicity of the inflating\,..,gases produced by the gas
generants. The built-in cat'a~yst is selected from the group
consisting of alkali metal, alkaline earth metal, and
transition metal salts of tetrazoles, bitetrazoles, and
triazoles, and transition metal oxides.
DETATLED DESCRIPTION OF THE PREFERRED EMBODIMENTISZ
In accordance with the present invention, the fuel
utilized in the nonazide gas generant is preferably selected
from compounds that maximize the nitrogen content of the
fuel and regulate the carbon and hydrogen content thereof
to moderate values. Such fuels are typically selected
from azole compounds, particularly tetrazole compounds
such as aminotetrazole, tetrazole, 5-nitrotetrazole,
5-nitroaminotetrazole, bitetrazole, and triazole compounds such
as 1,2,4-triazole-5-one or 3-nitro-1,2,4-triazole-5-one. A
preferred embodiment utilizes 5-aminotetrazole as the fuel
because of cost, availability and safety.
oxidizers generally supply all or most-of the-oxygen
present in the system. The oxidizer actively supports
combustion and further suppresses formation of CO. The
relative amounts of oxidizer and fuel used is selected to
provide a small excess of oxygen in the combustion products,
thereby limiting the formation of CO by oxidizing the CO to
carbon dioxide. The oxygen content in the combustion products
should be in the range of 0.1% to about 5% and preferably from
approximately 0.5% to 2%. The oxidizer is chosen from alkali
metal nitrates, chlorates and perchlorates and alkaline earth
metal nitrates, chlorates, and perchlorates. Strontium and
barium nitrates are easy to obtain in the anhydrous state and
are excellent oxidizers. Strontium nitrate and barium nitrate
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are most preferred because of the more easily filterable solid
products formed, as described hereinbelow.
A slag former is included in the gas generant in
order to facilitate the formation of solid particles that may
then be filtered from the gas stream. A convenient method of
incorporating a slag ,former into the gas generant is by
utilizing an oxidizer or a fuel which also serves in a dual
capacity as a slag former. The most preferred oxidizer which
also enhances slag formation is strontium nitrate, but barium
nitrate is also effective. Generally, slag formers may be
selected from numerous compounds, including alkali, alkaline
earth, and transition metal hydroxides, nitrates, chlorates,
and perchlorates, as well as alkali metal silicates, borates,
oxides, and carbonates, in addition to silicon dioxide,
alkaline earth metal oxides, and naturally and synthetically
manufactured magnesium and aluminum silicate compounds, such as
clay and talc.
In accordance with the present invention, the
built-in catalyst comprises an alkali metal salt, alkaline
earth metal salt, or transition metal salt of tetrazoles,
bitetrazoles and triazoles, or a transition metal oxide. The
catalyst, which is mixed directly into the gas generating
composition, promotes the conversion of CO and NOx to C02 and
N2. More specifically, metals, which are present in the form
of a salt of a tetrazole, bitetrazole, or triazole, or in the
form of a transitional metal oxide, catalyze two reactions.
For example, a typical primary reaction is as follows:
2C0 + 2N0 -~ 2C02 + NZ
It is also believed that the built-in catalyst also promotes a
secondary decomposition reaction, as follows:
2N0 -~ N2 + 0a
The amount of catalyst which is included in the gas
generating mixtures of the instant invention is preferably
within a range of about 5% by weight to about 15% by weight of
the gas generant mixture. Generally, -the fuel is present in
the gas,generants of the present invention in a concentration
of about 22% to about 50% by weight, the oxidizer is present in
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a concentration of about 30% to about 66% by weight, and the
slag forming compound is present in a concentration of about 2%
to about 10% by weight.
One skilled in the art will :readily appreciate the
manner in which the aforesaid combinations of ingredients are
combined to form the gas generant compositions of the present
invention. For example, the materials may be dry-blended and
attrited in a ball-mill and then palletized by compression
molding. The present invention may be exemplified by the
following representative examples wherein the components are
quantified in weight percent.
Example 1
A mixture of 5-aminotetrazole (5-AT) strontium
nitrate [Sr(N03)2], a copper salt of 5-AT, and clay is prepared
having the following composition in percent by weight:
28.62% 5-AT, 57.38% Sr(N03)2, 8.00% clay, and 6.00% of the
copper salt of 5-AT.
The above materials are dry-blended, attrited in a
ball-mill, and palletized by compression molding.
Example 2
A mixture of 5-AT, Sr(NOg)2, i~alc, and a zinc .salt
of 5-AT is prepared as described in Example 1 having the
following composition in percent by weight: 28.62% 5-AT,
57.38% Sr(N03)2, 6.00% talc, and 8.00% of the zinc salt
of 5-AT.
Example 3
A mixture of 5-AT, Sr(N03)z, a copper oxide, and talc
is prepared as described in Example 1 having the following
composition in percent by weight: 28.62% 5-AT, 57.38% Sr(N03)2,
6,00% copper oxide, and 8.00% talc.
Example 4
A mixture of 5-AT, Sr(N03)a, a zinc oxide, and clay is
prepared as described in Example 1 having the following
composition in percent by weight : 28 . 62% 5-AT, 57 . 38% Sr (N03) a,
8.00% zinc oxide and 6.00% clay.
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Example 5
A mixture of 5-AT, Sr(N03)z, a zinc oxide, and talc is
prepared as described in Example 1 h<~ving the following
composition in percent by weight: 28.62% 5-AT, 57.38% Sr(N03)z,
6.00o zinc oxide and 8.00o talc.
While the preferred embodiment of the invention has
been disclosed, it should be appreciated that the invention is
susceptible of modification without departing from the scope of
the following claims.
